Mycobacteria are significant human pathogens but are generally very slowly-growing organisms which may take weeks to identify even after being isolated in pure culture from clinical specimens. More rapid identification of such mycobacterial isolates would be clinically very useful, as the identification aids in determining the likelihood that the isolate is causing disease, helps determine whether isolation of the patient is necessary, and guides the choice of antibiotics. Several techniques for the rapid identification of these organisms have been reported, based on gas- liquid chromatography of cell wall extracts. We have adapted one of these procedures for use in our laboratory. The procedure is proving useful in the specific identification of some mycobacteria and in the assignment of others to certain species - groups. We have increased the specificity of the procedure, improved the inter-run precision, and expanded the data base on the basis of which the identifications are made.